1. Field of the Invention
The invention relates to an internal resistance estimation method of estimating an internal resistance of a secondary battery on the basis of a usage state of a secondary battery, an output control method for a secondary battery, which utilizes the internal resistance estimation method, and a vehicle that uses the output control method.
2. Description of Related Art
A nonaqueous electrolyte secondary battery is lighter in weight and higher in energy density than an existing battery, so the nonaqueous electrolyte secondary battery is favorably utilized as, for example, a vehicle-mounted high-power power supply. Particularly, in an application in which high-power density is required, a lithium ion secondary battery is favorably used. It is generally known that a secondary battery degrades as a result of usage of the secondary battery. A main cause of the degradation is presumably an increase in the internal resistance of a cell resulting from repeated charging and discharging of the secondary battery. As the internal resistance increases, the capacity of the secondary battery, of course, decreases, and a discharging rate, temperature characteristics, and the like, can decrease. For this reason, there is suggested a method of estimating the degree of degradation of a secondary battery at the time when the secondary battery is used (see, for example, Japanese Patent Application Publication No. 2014-149280 (JP 2014-149280 A) and Japanese Patent Application Publication No. 2012-185122 (JP 2012-185122 A)).
For example, JP 2014-149280 A describes an estimation method of estimating the degree of degradation of a secondary battery not during actual usage but from a charging or discharging curve under a certain condition without spending cost or time. In this estimation method, the degree of degradation is estimated on the basis of the amount of increase in the internal resistance of the secondary battery. If data measured at different temperatures are compared with each other, variations in analyzed results increase. Therefore, the internal resistance is divided into three components, that is, reaction resistance, ohmic resistance and diffusion resistance, these three components are corrected to those at a predetermined reference temperature, and then the degree of degradation is calculated. JP 2012-185122 A describes a method of calculating the degree of degradation of a secondary battery by taking an abnormality history into consideration in addition to the internal resistance value of the battery after usage. In JP 2012-185122 A, the degree of degradation is further accurately estimated by adding correction values of temperature and degree of charge based on the abnormality history to the normalized degree of degradation. The normalized degree of degradation is calculated by normalizing the internal resistance of each individual secondary battery to a predetermined reference temperature and a predetermined reference degree of charge.